It is known that titanium dioxide particles present as the pigment component in a coating material cause oxidative decomposition of the polymer on exposure to ultraviolet rays and moisture, which is known as whitening. In order to suppress this effect of titanium dioxide, it has been proposed to coat or dope titanium dioxide with compounds of chromium, silicon, aluminium, zinc, phosphorus or zirconium.
EP-A-0 268 918 describes a weathering-resistant pearl lustre pigment having a hydrated zirconium oxide coating on the titanium dioxide base pigment, this coating being obtained by hydrolysis of a zirconium salt in the presence of a hypophosphite.
EP-A-0 342 533 describes a weathering-resistant pearl lustre pigment having, on the titanium dioxide base pigment, a top layer which consists of hydrated zirconium oxide, obtained by hydrolysis in the presence of a hypophosphite, and a hydrated metal oxide. The metal oxide can be cobalt oxide, manganese oxide or cerium oxide.
The modified pearl lustre pigments possess sufficient dispersibility and weathering resistance in nonaqueous coating systems. However, they are unsuitable for use in water-thinable coating systems, since they cause the formation of microfine bubbles in the coating film which significantly increase light scattering and thus have an adverse effect on lustre and colour. In addition, the distinctness of image (DOI) is severely reduced and the regeneration capacity of the coating film is impaired.
U.S. Pat. No. 5,472,491 describes a pearl lustre pigment based on a platelet-form substrate coated with metal oxides and on a top layer which is located on the metal oxide layer and consists of silicon dioxide, at least one further metal hydroxide or metal hydrate of the elements cerium, aluminium or zirconium, and at least one organic coupling reagent. This coupling reagent can be a zirconium aluminate, a metal-acid ester or an organofunctional silane. The coupling reagents must be hydrolysed before binding to the pigment surface. When two or more compounds are used, problems occur as a result of different rates of hydrolysis. Moreover, not more than 60% of the coupling reagents added can be bound to the pigment surface, as a result of which it is necessary to make a corresponding increase in the material employed. The non-coupled fractions remain in the reaction medium and impair the filterability of the pigment. For the binding of the pigment to the coating binder, however, it is necessary for different functional groups to be present on the pigment surface.